Printed circuit boards (PCBs) are critical components for many electronic devices and have evolved from conventional single-layer boards to multi-layer boards which employ multiple circuit layers to enable various circuit designs and applications. Recently, high density interconnection (HDI) technology has been developed, which uses through holes and blind holes to achieve inter-layer circuit connections and signal transmission of multi-layer boards. Inevitably, multi-layer boards tend to have greater thickness. In order to meet the demands of compact, light-weight and conveniently portable electronic products, such as smart phones, PCB makers begin to use thinner glass fiber cloth to make prepregs. Commonly used thin glass fiber cloth includes 1017, 1027, 1037, 106 and 1067 types. Reduction of overall thickness of the PCB products is expected by reducing the thickness of prepregs.
In addition, trace pattern designers have begun studying the ways of using layer reduction to reduce the number of layers of multi-layer boards, in hopes of retaining the signal transmission functionality provided by the original number of layers with reduced overall thickness.
The primary advantages of layer reduction include reducing the overall thickness of PCBs and correspondingly reducing the overall thickness of end products, such as smart phones. In addition, it is well-known that circuit board production involves many different processes, at least including cutting, baking, mechanical drilling (alignment holes), fabrication of inner circuits, layer build-up lamination, laser drilling, exposure and development, fabrication of inner circuits, layer build-up lamination, laser drilling, exposure and development, fabrication of inner circuits and so on, and the formation of each additional circuit layer requires the repetition of several processes that are cost-ineffective, including fabrication of inner circuits, layer build-up lamination, laser drilling, exposure and development, etc. Therefore, the implementation of layer reduction design would greatly save the production costs of multi-layer PCBs.
However, when the thickness of PCBs is reduced, a problem commonly faced by the industry is the higher precision and narrower trace demand, which are prerequisites to meet the specification of characteristic impedance, resulting in substantial decrease of the yield rate of the production process and significant increase of costs. Therefore, there is a need for a PCB design which can maintain excellent signal transmission properties and high yield rate of the production process when the thickness and/or number of layers of PCBs is reduced.